Transfer learning to train only for a new class while not affecting the predictions of the other class
Even if you want to re-train your model for just one new class you will have to prepare your training data such that it includes all or most of the classes which you want to predict. Most of the times last two layers of a network have the data of number of labels which are to be predicted and that should always be sum of the number of classes you already trained on and the number of classes you want to add for training.